Software defined radios offer the promise of wider capability in a smaller hardware footprint. This has become more important in cellular handsets as customers demand a wider variety of radio capabilities in a single mobile terminal while still expecting the handset to fit neatly in a pocket. These software-defined radios operate over a wide range of frequencies to implement multi-mode and multi-band functionality for cellular and data connectivity applications. However, current technological limitations have prevented realization of widely tunable radio-frequency (RF) front-end circuits for multi-band radios. Such multi-band radios are increasingly required by certain cellular radio access technologies (RAT) such as evolved Universal Terrestrial Radio Access Network (E-UTRAN, also known as long term evolution or LTE).
In the typical radio frequency (RF) front end, the first component after the antenna is a multi-band switch/diplexer followed by a duplexer. In existing handsets, many duplexers may be used depending upon the number of respective receive/transmit bands and their corresponding bandwidths. The duplexer provides isolation between a transmitter and a receiver, to thereby enable the same antenna to be used for both transmitting and receiving. The transmit signal has a very high power, up to 33 dBm, while the receiver is required to pick up signals as low as −109 dBm. Therefore, the transmitted signal has to be isolated from the received signal even though the transmitter and the receiver may be operating in different frequency bands. In addition to leakage from the transmitter, to the receiver, there may be other blocking signals in the vicinity of the desired signal that also need to be suppressed. Typically, the transmit and receive frequencies are not widely separated, so in practice, about 50 dB of isolation is required between the transmitter and the receiver. This degree of isolation is usually provided by means of a surface acoustic wave (SAW) filter that operates over a limited band of frequencies.
While a SAW filter may be sufficient for operation across a limited frequency band as occurs, for example, in UTRAN, the SAW filter is too cumbersome, costly, large, narrowband and inefficient for use in more advanced RATs such as LTE. SAW filters provide a significant insertion loss on the order of 2 or 3 dB which negatively impacts the sensitivity and noise figure of a receiver. Likewise, a significant portion of the transmitted output power will be dissipated as heat in the SAW filter. The SAW filter has several drawbacks which render it not ideally suited for use in the RF front end of a software defined radio. In addition to the aforementioned drawbacks, SAW filters may occupy too much of the available area on a printed circuit board which impacts the form factor of the host mobile device. Moreover, SAW filters may not adequately suppress undesired signals close in frequency to the desired signal, yet this reception condition is fairly commonplace in multi-band and multi-mode smart-phones.
A single radio transceiver is desirable both for cellular and data connectivity applications. With the opening of several new frequency bands for cellular applications, a reconfigurable multi-standard radio is required to work over a wide range of frequencies. Currently, if a handset is configured to implement more than one standard such as GSM, WCDMA, LTE, or WiFi, the handset may be equipped with separate RF transceivers for implementing each of the standards. Moreover, a multi-standard radio may be required to receive signals which have two or more different bandwidths.
Other relevant teachings include U.S. Pat. No. 3,603,898 by John H. D. Chelmsford et al.; UK Patent No. 1,341,182 by Michael A. Kaufman; and a paper by Milad Darvishi et al. at section 21.1 (Analog Techniques, pages 358-359) of 2012 IEEE International Solid State Circuits Conference.